1. Field of the Invention
This invention relates in one embodiment to a method of recovering crude oil from subterranean formations wherein a displacing fluid is injected into the formation through at least one injection means to displace crude oil and is driven by means of a drive fluid toward at least one production means; more specifically, this invention in that embodiment relates to a secondary and/or tertiary recovery method for recovering crude oil from a subterranean formation wherein an improved drive fluid is employed to drive through the formation a displacing fluid capable of displacing the crude oil in the formation to the production means. In a further embodiment of the present invention a waterflood of improved sweep efficiency is provided through the use of a specific mobility control agent.
2. Description of the Prior Art
Knowledge is widespread in the oil industry that the so-called "primary recovery" techniques, which include natural flow, gas lifting, gas repressurization and pumping methods, leave substantial quantities of oil in oil-bearing reservoirs. Recognition of the large amount of oil remaining in many oil-producing reservoirs has led to the use of so-called "secondary and tertiary recovery" techniques which have as their primary purpose the economical recovery of additional quantities of the oil known to be present in the reservoir.
Probably one of the more common secondary recovery techniques is the so-called "waterflooding" in which aqueous fluids are injected at one or more points in the reservoir at pressures sufficient to force the fluids out into the reservoir and toward a spaced production well or wells. This, in effect, displaces oil from the pores of the reservoir and drives the oil ahead of the water front.
However, such techniques, e.g. waterflooding, are only advantageous when the cost of injecting water and necessary chemical modifiers is less than the value of the oil recovered. As a result the displacement efficiency of waterfloods or the like has been the determining factor of whether such a technique will be used.
Generally, the difficulty with waterfloods is that the small pores and capillaries of the reservoir contain hydrocarbons (oil and/or oil and gas) which are generally water immiscible. The existence of high interfacial tensions between the boundary of the water and hydrocarbons seriously impedes the ability of the water to displace oil trapped in the reservoir by capillarity.
Since in many oil-bearing reservoirs the oil tends to be trapped within the pores of the rock formations by capillarity, merely forcing water therethrough will not displace this trapped oil. However, a sufficient reduction in the interfacial tension between the water and the oil will increase the amount of oil that will be displaced by the water. Thus, various aqueous surfactant systems have been proposed for use in waterflooding processes for recovering oil, the surfactants having the ability to reduce the interfacial tension between the oil and water.
Many aqueous systems containing surfactants have also been proposed in which the effective oil recovery is improved by treating the formation with a liquid which contains an oil-solubilizing, aqueous solution of surfactant micelles that are combined with molecules of an amphiphilic organic compound of low water solubility; the amphiphilic material being capable of swelling the surfactant micelles and causing the aqueous solution to solubilize a significant proportion of oil. In using such systems oil is recovered by injecting an aqueous liquid to drive the surfactant system and the oil towards the production means.
A fluid which has been used in secondary and tertiary oil recovery is one which employs a solvent that is miscible with both the connate oil and with the flood water. This fluid is introduced into the formation ahead of the flood water. Alcohols and various other organic solvents have been proposed as suitable miscible agents for use in the so-called miscible flooding operation. Methods utilizing hydrocarbons as miscible fluids are extremely successful with regard to the displacement efficiency of the crude oil, particularly the ability to displace essentially all of the crude oil from the formation contacted. However, the volumetric sweep efficiency of these flooding media is a disadvantage. The hydrocarbons inherently finger and channel to a great extent and a large portion of the reservoir is bypassed although the volume of injected hydrocarbons may be great. This is economically disadvantageous.
A typical miscible flooding process is the miscible slug process. This is a process which consists of injecting a limited quantity of fluid, e.g. propane, LPG, or similar solvent, into an oil reservoir and pushing this liquid bank toward producing wells with another fluid injected subsequently. The injected fluid being miscible with reservoir oil results in efficient displacement of the oil.
More recently, attention has turned toward the injection of a microemulsion in the form of a microemulsion slug. Such microemulsion can be defined as a stable transparent or translucent micellar solution of oil and water that may contain one or more electrolytes, and a surfactant, optionally containing one or more cosurfactants. The oil, water and surfactant are essential components of the microemulsion with the surfactant being present in an amount greater than the critical micelle concentration so as to form the desired microemulsion. The microemulsion can be a water-external microemulsion, an oil-external microemulsion or a microemulsion in which there is no discernible external phase.
Regardless of the type of displacing fluid which is employed in the secondary and/or tertiary recovery process, one or more slugs of the displacing fluid are driven through the formation by means of a driving fluid. To eliminate a "fingering" effect of the displacing fluid slug through the formation, the displacing fluid slug is generally provided with a mobility control agent. Likewise, in order to eliminate an unwarranted fingering effect between the displacing fluid and drive fluid, it has been proposed to thicken the drive fluid so that the mobility of the drive fluid is substantially equal to or less than the mobility of the displacing fluid slug.
Various materials have been proposed for use as thickening agents or mobility control agents to increase the viscosity of the flood water. Such materials include fatty acid soaps, alginates, sucrose, dextran, amines, glycerine, and a number of water-soluble polymers. The materials which have been found commercially satisfactory fall into two general categories, the first being the natural polymers such as polysaccharides with the second being a class of polyacrylamides, specifically partially hydrolyzed polyacrylamides. Commercially available thickeners falling within the foregoing groups include a material sold by Kelco Industries under the name of "Kelzan XC," a polysaccharide, and materials sold by Dow Chemical under the name "Pusher," a partially hydrolyzed polyacrylamide. While these materials are effective in thickening the aqueous flood and decreasing the mobility thereof, they have certain serious drawbacks, particularly when utilized to drive a microemulsion slug and in waterflood applications.
For example, the polysaccharides, while showing sufficient thickening characteristics, have the disadvantage of poor filtration characteristics; particularly in saline water. The polyacrylamide materials are also disadvantageous in that they show permanent shear degradation effects even at low shear rates. Materials of the foregoing types, cannot be advantageously employed as driving fluids for microemulsion slugs since both types of material show very poor phase behavior with microemulsions, the microemulsions quickly breaking down into multiphase systems, thereby shortening the efficient miscible displacement of single phase microemulsions. Accordingly, the art has long sought a drive fluid which can be effectively employed with all typically employed displacing fluids which eliminates the disadvantages of the materials proposed heretofore.